While machines and instruments are operated, their parts make sliding contact or rotating contact with each other, so that their metal surfaces are worn. Accordingly, a lubricating oil has an important role to restrain wear of the contacting parts of machines and instruments.
However, a base oil of a lubricating oil (e.g., a vacuum distillation oil obtained from atmospheric distillation residual oil and a synthetic oil) of itself cannot exhibit a number of characteristics that are specifically required in other applications of lubricating oil compositions such as lubricating oils for an internal combustion engine and a driving system.
Accordingly, an additive plays extremely an important role in order to improve wear resistance of the lubricating oil and extend a lifetime of such instruments.
A known additive to improve wear resistance is ZnDTP (Zinc Dialkyldithiophosphate). ZnDTP is not only excellent in extreme-pressure property and wear resistance but also exhibits antioxidant capacity, anticorrosive property and load resistance capacity, whereby ZnDTP has been widely used for an engine oil as a so-called multi-functional additive.
However, although exhibiting excellent performance on one hand, ZnDTP itself degrades to generate an acid material such as sulfuric acid or phosphoric acid, so that such the acid material reacts with a base component contained in the engine oil, thereby causing a decrease in base-number thereof and shortening the lifetime of the engine oil. Accordingly, an extreme pressure agent and an antiwear agent as an alternative of ZnDTP have been desired.
For example, zinc dialkylphosphate having a specific structure is known for providing wear resistance as well as being excellent in a base number retention property under high temperature and oxidative conditions such as in an engine (see Patent Documents 1 to 3). Moreover, a specific phosphate ester compound is also known for providing excellent extreme-pressure property and wear resistance under high temperature and high load when added to an engine oil (see Patent Document 4).    Patent Document 1: JP-A-2002-294271    Patent Document 2: JP-A-2004-035619    Patent Document 3: JP-A-2004-035620    Patent Document 4: JP-A-2006-063248